1. Field of the Invention
The invention relates to devices producing images by the effect of excitation of a luminescent screen. It relates more particularly (but not exclusively) to the cathodoluminescent screens of radiological image intensifier tubes (called RII tube for short).
Taking the case of RII tubes for example, these tubes are used mainly in medical imaging, that is to say in the context of radiodiagnosis, where they produce a visible image which conveys the radiological image of a patient.
2. Discussion of Background
FIG. 1 diagrammatically shows conventional radiodiagnosis equipment. This equipment includes a source SX of X-rays delivering radiation RX to which a patient P is exposed.
On the other side of the patient P, that is to say opposite the source SX, the X radiation carrying a radiological image is picked up by an RII tube.
The RII tube generally comprises a vacuum enclosure 2 closed at one end by an entry window FE through which the X radiation penetrates. This X radiation then encounters an entry screen EE, the function of which is to translate the intensity of the X radiation incident at each point of its surface into a number of electrons (not represented).
To this end, the entry screen EE generally comprises a scintillator SC associated with a photocathode PhC.
The scintillator converts the X radiation into visible photons which are themselves converted into electrons by the photocathode.
A set of electrodes DE accelerates these electrons and focuses them onto a cathodoluminescent screen called exit screen ES. The exit screen ES is arranged in proximity to an exit window FS or exit wall situated at the second end of the RII tube, opposite the entry window FE.
The impact of the electrons on the cathodoluminescent screen ES makes it possible to reconstitute the image (amplified in terms of brightness) which, at the outset, was formed on the surface of the photocathode PhC of the entry screen.
The exit window FS is a transparent component generally made of glass (or possibly also consisting of an optical-fibre device), which can be made, for example, from a component fixed onto the envelope of the enclosure 2, or may even constitute a part of this envelope. The exit window FS carries the cathodoluminescent screen ES which, in general, consists of a layer of phosphor material. Under these conditions, the image, in visible light, formed by the cathodoluminescent screen ES is visible from outside the RII tube, through the exit window FS.
The image delivered by the cathodoluminescent exit screen ES is generally viewed via an optical device DO, arranged outside the RII tube, centred, for example, on a longitudinal axis 5 of the RII tube, around which axis the cathodoluminescent screen ES is also centred.
This image may possibly be distributed by the optical device Do, on the one hand, towards various image detectors, such as, for example, cinematographic and photographic imaging cameras marked 6, 7, respectively, arranged on either side of the optical device Do on a second axis 8 perpendicular to the axis 5 of the tube, and, on the other hand, towards an image detector consisting of a television imaging camera CT.
The television camera CT is linked to a visual display device DV capable, in "direct" mode, of displaying the image which is delivered in the form of electrical signals by the television camera CT (in the case of radioscopy). In the example represented, the camera CT is also linked to a signal acquisition and processing device ATS which can store and process the signals, in digital form, relating to the image (in the case of digital radiography) and possibly correct the image displayed by the visual display device DV.
Equipment like that shown in FIG. 1 is currently used successively in fluoroscopy or radioscopy mode, and in digital radiography mode. However, these two modes pose different problems.
In the case of digital radiography, the doses of X-rays are often significant (and the duration of application of the radiation is very short (a few milliseconds)). The repetition rate of the images is variable according to the applications, from a few images per second up to television frequency, and the image resolution sought is the highest possible.
In the case of fluoroscopy or radioscopy, the radiological imaging system shown in FIG. 1 operates at the television frequency (25 or 30 images/s), with X-radiation doses which are much weaker, however the resolution of the details sought is lower. Due to the weak doses of X radiation used, the spatial-temporal fluctuation (quantum fluctuation of the X radiation) is perceptible in the video image delivered via the radiological imaging system. In order to limit this fluctuation, and enhance the quality of the image, it is necessary to perform time-based integration of the luminous intensity at each point of the image, in order to obtain a "smoothing" of the apparent time-based noise. Obviously, a practical compromise exists between an integration time which is sufficient to reduce the noise, and an integration time which is short enough not to introduce "blurring" around the image of the moving organs (smearing effect).
In order to obtain attenuation of the perceptible noise, in fluoroscopy, several solutions are currently employed:
a--Use of a television imaging camera equipped with a remanent tube. PA1 b--The use of a remanent luminescent tube at the output from the image intensifier tube. PA1 c--The use of image processing, on the basis of a partial cumulation of the video signal from each point of the image, for several successive frames.
As far as the first two solutions (a) and (b) are concerned: they exhibit the drawback of optimizing the radiological imaging system for radioscopy, to the detriment of its use in digital radiography. In fact, in digital radiography, it is desirable for the remanence (persistence of the luminescence) to be as low as possible, particularly in order to reduce the "blurring" which this remanence would introduce for the observation of moving organs (heart, for example) or the introduction of opacifying agents. It should be noted that, at the present time, television imaging cameras are today increasingly equipped with photosensitive sensors of the CCD type (from "Charge Coupled Device") which introduce very low remanence, and which are therefore able to pick up images in "high-speed" mode, that is to say in digital radiography mode, but which, without digital integration, produce images which are too noisy in fluoroscopy mode.
As far as the third solution (c) is concerned: it entails cumbersome and expensive facilities, especially for implementing a high-resolution image memory.